The present invention relates to a surface acoustic wave band-pass filter and, particularly, to such a filter constructed with a plurality of transducers each comprising comb type interdigital electrodes operating at a frequency in VHF or UHF frequency range.
Japanese Patent Application Laid-Open No. 202114/1982 discloses an example of a conventional surface acoustic wave band-pass filter and FIG. 2 of this application corresponds thereto.
In FIG. 2, the conventional surface acoustic wave band-pass filter comprises a substrate, a first surface acoustic wave device A arranged on the substrate, a second surface acoustic wave device B arranged on the substrate symmetrically to the first surface acoustic wave device A about a line C extending between the devices A and B in parallel to surface wave propagating direction, an electric signal source 1, an output side load 2 and an input side load 3. The first surface acoustic wave device A includes a plurality (five in this case) of first electro-acoustic transducers 14-1 to 14-5 of comb shaped interdigital electrode type arranged repetitively in the surface wave propagating direction and a plurality (four in this case) of first acousto-electric transducers 15-1 to 15-4 each arranged between adjacent ones of the first electro-acoustic transducers 14-1 to 14-5. The second surface acoustic wave device B includes a plurality (five in this case) of second acousto-electric transducers 16-1 to 16-5 of comb shaped interdigital electrode type arranged repetitively in the surface acoustic wave propagating direction and a plurality (four in this case) of second electro-acoustic transducers 17-1 to 17-4 each arranged between adjacent ones of the second acousto-electric electro transducers 16-1 to 16-5.
As shown in FIG. 2, the first acousto-electric transducers 15-1, 15-2, 15-3 and 15-4 and the second electro-acoustic transducers 17-1, 17-2, 17-3 and 17-4 are identical to each other and arranged symmetrically about the line C.
In the surface acoustic wave band-pass filter shown in FIG. 2, surface acoustic wave is trapped therein due to the repetitive arrangements of the first acousto-electric transducers 15-1 to 15-4 of the first surface acoustic device A and of the second electro-acoustic transducers 17-1 to 17-4 of the second surface acoustic wave device B, so that loss caused by surface acoustic wave leakage from opposite end portions of the filter structure is minimized. However, since this structure is intended to prevent surface acoustic wave from leaking away therefrom, there may be transmission of off-band power from an input to an output of the filter, resulting in a degraded attenuation for off-band signal.
An article, M. Hikita et al., "Phase Weighting for Low Loss SAW Filters", Proc. IEEE, Ultrasonics Symp., Nov. 1980, pp. 308-312, discloses filter designs based on a new phase weighting method, according to which the attenuation problem of off-band signal is solved to some extent. Although this proposal is effective to provide an off-band attenuation to some extent, it is still not enough.
As other related prior arts, U.S. Pat. No. 4,365,220 issued to M. Hikita who is a co-inventor of this application discloses a surface wave device using uniform phase weighting, an article, M. Hikita et al., "HIGH PERFORMANCE SAW FILTERS WITH SEVERAL NEW TECHNOLOGIES FOR CELLULAR RADIO", Proc. IEEE, Ultrasonics Symp., Nov. 14-16, 1984, pp 82-92 and U.S. Pat. No. 4,468,642 issued to M. Hikita disclose band-pass filters composed of symmetrically arranged, equally phase-weighted interdigital tansducers.